Heretofore, whenever flame retardants have been added to various thermoplastic polymers such as polypropylenes, various physical properties have been affected such as reduced impact strength and enhanced flexural modulus. However, various polypropylenes containing flame retardants exhibit processibility problems, especially the processing of large, injection-molded parts. Such flow problems have been somewhat alleviated by increasing the processing temperature. However, increased processing temperatures cause instability problems such as undesirable color changes, as well as major problems such as decomposition resulting in polymer breakdown and inferior properties.
Non-flame retardant polyolefin compositions have had their molecular weight distribution altered.
For example, U.S. Pat. No. 3,953,655, to Steinkamp et al, relates to modified polymers, particularly polyolefins, which have improved flow and in some instances improved adhesion properties over that of a polymeric, e.g., polyolefin base stock used as a starting material, and are produced by a controlled reaction often involving degradation in an extruder, in which an initiator is injected under conditions of either maximum distribution or intensive mixing wherein appreciable rheological changes, i.e., molecular weight distribution, in said base polymer occur. In some embodiments, monomers are also grafted to said base stock, during said degradation process. In such instances, grafted polymers with high melt flow properties are obtained.
U.S. Pat. No. 4,451,589, to Morman, relates to thermoplastic polymers with improved processibility resulting from initial partial degradation of high molecular weight polymers using a chemical prodegradant present in excess of the amount reacted during pelletization. This class of polymers includes polymers and copolymers of propylene and butylene. After pelletizing, the polymer, including unreacted prodegradant, can be safely handled and shipped without difficulty. When remelted by extruding or the like, the prodegradant in the pellets reacts, further reducing the molecular weight as well as narrowing the molecular weight distribution of the polymer to a point where high capacity production of quality fibers and extruded products can be obtained.
British Patent No. 1,042,178, to Trieschmann et al, relates to a process for narrowing the molecular weigh distribution of polyolefins by degradation at temperatures between the melting point and the temperature at which purely thermal degradation of the polyolefins occurs, by means of high shear gradients.